Audio encoder with selectable L/R or M/S coding

1. An encoder system for encoding a stereo audio signal having an original left channel and an original right channel, the encoder system comprising: a parameter estimator for determining one or more stereo parameters based on the stereo audio signal, the one or more stereo parameters describing a stereo image of the stereo audio signal; a downmixer for generating a downmix signal and a residual signal based on the stereo audio signal, the residual signal indicating an error associated with representing the stereo audio signal by the downmix signal and the one or more stereo parameters, wherein the residual signal covers only a part of an audio frequency range of the stereo signal; a selector that analyzes the stereo audio signal and selects: left/right perceptual encoding, or mid/side perceptual encoding, in a time variant manner; a high frequency reconstruction encoder for generating high frequency reconstruction parameters, the high frequency reconstruction parameters including a spectral envelope of a high band portion of the original left channel and the original right channel; and a bitstream generator for creating an audio bitstream containing a coded representation of the stereo audio signal, the one or more stereo parameters, and the high frequency reconstruction parameters.

2. The encoder system of claim 1 wherein the selector analyzes by determining which stereo coding mode would more efficiently code the stereo audio signal.

3. The encoder system of claim 1 wherein the selector analyzes by applying both mid/siding stereo coding and left/right stereo coding and selecting the stereo coding mode based on an estimated entropy for each stereo coding mode.

4. The encoder system of claim 1 wherein the stereo parameters are time and/or frequency variant.

5. A method for encoding a stereo audio signal having an original left channel and an original right channel, the method comprising: determining one or more stereo parameters based on the stereo audio signal, the one or more stereo parameters describing a stereo image of the stereo audio signal; generating a downmix signal and a residual signal based on the stereo audio signal, the residual signal indicating an error associated with representing the stereo audio signal by the downmix signal and the one or more stereo parameters, wherein the residual signal covers only a part of an audio frequency range of the stereo signal; analyzing the stereo audio signal and selecting, based on the analyzing: left/right perceptual encoding, or mid/side perceptual encoding, in a time variant manner; generating high frequency reconstruction parameters, the high frequency reconstruction parameters including a spectral envelope of a high band portion of the original left channel and the original right channel; and generating an audio bitstream containing a coded representation of the stereo audio signal, the one or more stereo parameters, and the high frequency reconstruction parameters.

6. The method of claim 5 wherein the analyzing includes determining which stereo coding mode would more efficiently code the stereo audio signal.

7. The method of claim 5 wherein the analyzing includes applying both mid/siding stereo coding and left/right stereo coding and selecting the stereo coding mode based on an estimated entropy for each stereo coding mode.

8. The method of claim 5 wherein the stereo parameters are time and/or frequency variant.

9. A non-transitory computer readable medium comprising instructions that when executed by a processor cause the processor to perform the method of claim 5 .